1. Field of the Invention
The present invention relates to the preparation of bleached chemithermomechanical wood pulp.
2. Description of the Prior Art
"Chemithermomechanical pulps", hereinafter CTMP pulps, are known to this art as pulps produced by treating (digesting) a lignocellulosic material, generally wood in the form of chips, with one or more chemical agents, combined with the operations of heating and mechanical separation of fibers.
CTMP pulps have a unique industrial worth as they constitute a favorable compromise between purely mechanically disintegrated pulps and purely chemically digested pulps.
For example, they are produced in a yield, weight of pulp relative to the weight of the starting materials in the dry state, of generally greater than 85%, most typically at least equal to 90%. In this respect they very closely approximate pulps of purely mechanical origin.
In the combined operation indicated above of heating, chemical treatment and fiber separation, the chemical treatment may be carried out either before, during or after the fiber separation.
By "chemical treatment" is intended that operation, over the course of which the lignocellulosic material is digested with a sulfite, notable sodium sulfite, Na.sub.2 SO.sub.3, or a bisulfite, notably sodium bisulfite, NaHSO.sub.3, or, more generally, a mixture of sulfur dioxide, SO.sub.2, and sodium hydroxide, NAOH, at a temperature equal to or greater than 100.degree. C. under saturation water vapor pressure. Such sulfite, bisulfite or mixture thereof will hereinafter collectively be designated as the "sulfite". The chemical treatment potentially includes a conventional impregnation of the lignocellulosic material with a solution of the selected reagents.
The temperature at which the treatment is carried out generally does not exceed 200.degree. C. and preferably ranges from about 120.degree. to 160.degree. C.
The treatment medium is at an initial pH preferably ranging from 6 to 12.5.
Its consistency, concentration by weight of the pulp expressed in the dry state in the medium, typically ranges from 10% to 40%, most usually from 15% to 30%.
The duration of the treatment depends on the selection of other process parameters, but generally does not exceed 1 hour.
Expressed in terms of SO.sub.2, the amount of the sulfite ranges, for example, from approximately 0.1% to 10%, most typically from 0.5% to 3%, with the percentages being given by weight relative to the weight of the lignocellulosic material in the dry state (this convention will also be used in the description which follows, unless otherwise indicated).
Certain chemical agents may be used in the treatment together with the sulfite, for example complexing or sequestering agents, such as diethylenetriaminepentaacetic (DTPA) or ethylenediaminetetraacetic (EDTA) acid used in the form of the sodium salts in amounts generally ranging from 0.1% to 1%.
The combination, as indicated above, of the chemical treatment with the operations of heating and mechanical grinding generally is carried out, in actual practice, with two successive (two-stage) refining operations, to render the pulp usable in conformity with paper industry requirements.
Specifically as regards the production of CTMP pulps, see the text by James P. Casey, Pulp and Paper Chemistry & Chemical Technology, 3rd edition, Vol. I, 1980, in particular pages 241-245, 213-219-229, the text Pulp & Paper Manufacture, Vol. 2, 1987, "Mechanical Pulping", in particular Chapters VIII D and XI, and U.S. Pat. No. 4,718,980, in particular FIG. 1.
By "bleached CTMP pulps" are intended the above CTMP pulps, after they are bleached by means of hydrogen peroxide, H.sub.2 O.sub.2, in an alkaline medium.
In the known process for the preparation of bleached CTMP pulps, prior to bleaching with hydrogen peroxide, chemical agents such as sulfite ions must be removed as completely as possible, as it is known that they effect a consumption of H.sub.2 O.sub.2 which is detrimental to the bleaching process, as reported, for example, by H. Kruger & H. U. Suss, TAPPI Proceedings, International Sulfite Pulping Conference, pp. 143-148 (1982).
In actual practice, the paste is carefully washed prior to bleaching. Conventionally, this is carried out, for example, by means of a potentially repeated sequence of dilution and reconcentration of the pulp.
When an operation of screening/cleaning of the refined pulp is carried out, as is generally the case, the dilution should adjust the consistency to a range of values as low as approximately 0.5% to 2%. Concerning washing and screening/cleaning, see the above Casey reference, pages 228-231, 363-365, 447-452, and the above Pulp & Paper Manufacture text, in particular Chapters XIII-XVIII.
Reconcentrating the pulp to be bleached to a consistency equal to at least about 10%, from very low values, is an operation required by the known process, not only to efficiently eliminate the chemical agent or agents considered to be undesirable in bleaching, but also such that the hydrogen peroxide will have a satisfactory effectiveness in the absence of these compounds.
The bleaching of the CTMP pulp by means of hydrogen peroxide in an alkaline medium is typically carried out by introducing an amount of hydrogen peroxide of from approximately 0.5% to 10%, in the presence of about 1% to 6% of a sodium silicate solution having a density of 1.33, at a pH of from approximately 9 to 11 and at a temperature of from about 40.degree. C. to 100.degree. C. for from about 0.5 to 2 hours, at a consistency of approximately 10% to 30%. The bleaching bath may also contain certain additives, principally one or more sequestering or complexing agents, such as, for example, DTPA, in amounts generally ranging from approximately 0.1% to 1%.
After bleaching, the pulp is preferably treated with an acid, such as, for example, SO.sub.2, to stabilize the bleaching, prior to being diluted with water to a very low consistency, for example on the order of 1%, to permit its transport, its storage and its use in papermaking operations.
The existing process for the preparation of bleached CTMP described above and considered in relation to the manufacture of paper has certain serious disadvantages relative to economy and/or pollution:
(i) The production of a very large amount of aqueous effluents, about 100 tons per ton of paper; and PA1 (ii) The necessary reconcentration of the pulp prior to bleaching, which involves the use of expensive equipment, filters or a press for the pulp, for example, in spite of which it remains difficult to obtain a high consistency (which is known to favor the bleaching action of the hydrogen peroxide).
The aqueous effluents, which originate essentially in the operations of washing, screening/cleaning, the separation of water of the final dilution prior to or during the production of paper (depending on whether or not this production is integrated with the preparation of the pulp), which contain practically no sulfite, but are still high in pollutants, are necessarily recycled and in particular are used as a washing agent for the pulp, in order to eliminate the sulfite prior to the bleaching with hydrogen peroxide.
These effluents will hereinafter be designated as "clear industrial water."
In spite of the effective washing of the bleached CTMP pulp produced, its degree of whiteness remains appreciably less than that obtained by the use of natural demineralized and not recycled water, an application that unfortunately is not feasible from an economic standpoint alone.